A dental prosthetic of ceramics comprising chiefly silica has been used in the dental therapy in order to restore a decayed tooth or a broken tooth, or to restore a prosthetic.
The dental prosthetic of ceramics is, usually, joined to an object that is to be joined, such as tooth by using a dental cement such as resin-reinforced glass ionomer cement, resin ionomer cement or resin cement. These dental cements are all adhesives comprising chiefly a polymerizable monomer and a polymerization initiator, and require, in advance at the time of adhesion, the application of a primer (primer for ceramics) comprising chiefly a silane coupling agent onto the junction surfaces of the dental prosthetic of ceramics in order to enhance the adhering property. For instance, there have been proposed dental primers comprising, as a constituent component, an acidic organophosphorous compound having at least one radically polymerizable olefinic double bond in a molecule thereof (patent documents 1 and 2). In these dental primers, the acidic organophosphorous compound works as a catalyst for activating the silane coupling agent and for accelerating the condensation of silanol group on the surfaces of the ceramics. There has, further, been proposed a dental primer containing an acidic group-containing polymerizable monomer, polyvalent metal ions and a volatile organic solvent. However, this primer is applied to a cured body of resin and must contain water (patent document 3).
As the dental ceramics, on the other hand, there have been widely used metal oxide ceramics such as zirconia or alumina serving as dental prosthetic materials of all-ceramics featuring a larger strength. Even if treated with a general primer for ceramics comprising chiefly the silane coupling agent, however, these metal oxide ceramic materials are not capable of maintaining a sufficiently large adhering property for extended periods of time in a sever environment in an oral cavity. The reasons are attributed as described below. The dental ceramic material that is generally used chiefly comprises silica as described above and has many silanol groups on the surfaces thereof exhibiting favorable reactivity with the silane coupling agent. On the other hand, the metal oxide ceramics such as zirconia or alumina has almost no silanol group or only very few silanol groups, and does not react with the silane coupling agent. Therefore, it has been strongly desired to develop a primer for pre-treating the surfaces of metal oxide ceramics, having a new action and mechanism that meet properties of the metal oxide material in order to improve the adhering property.
As the primer for metal oxide ceramics, there has been proposed a dental adhesive composition containing a phosphonic acidic group-containing (meth)acrylate type monomer (patent document 4). There has been disclosed that the composition may contain a coupling agent in addition to the phosphonic acidic group-containing (meth)acrylate type monomer. Concretely, there have been disclosed titanate type coupling agents such as isopropyltriisostearoyl titanate, isopropyldimethacryloylisostearoyl titanate; and aluminum type coupling agents such as acetoalkoxyaluminum diisopropylate. There has, further, been proposed a primer for ceramics containing a (meth)acrylic acid ester type polymerizable monomer of a specific structure, such as 2-acetoacetoxyethyl methacrylate (patent document 5). The present inventors have tested the above novel primers for metal oxide ceramics for their adhering forces and have discovered that though all of them exhibit large initial adhering forces, their durability of adhesion is not sufficient. In particular, their adhering property decreases after the use in a severe environment in an oral cavity.